The present invention relates to a device and a process for controlling at least one aerodynamic elevator surface of an aircraft, during a takeoff phase.
It is known that, to achieve aircraft takeoff, the pilot accelerates this aircraft over the runway, until it reaches a speed Vr (at a time tr) at which he actuates a control member of the aircraft, for example a ministick. This actuation brings about the movement of the aerodynamic elevator surfaces, of the elevators type, this having the effect of raising the nose of the aircraft, the nose gear of which lifts off the ground, the aircraft continuing to run over the runway by means of its main gear. The aircraft is then still in the acceleration phase, its longitudinal axis being inclined by a pitch attitude angle xcex8 with respect to the runway. When the aircraft reaches a speed Vlof, at a duration T after the actuation of the control member, the wheels of the main gear leave the runway in turn and takeoff is achieved.
However, before takeoff is achieved, while the aircraft is running on the runway, there is a risk of the aircraft touching said runway with the rear part of its fuselage if the pitch attitude angle exhibits too high a value.
Generally, the change in the attitude angle xcex8 between the times tr and tr+T satisfies a particular control law, according to which said attitude angle xcex8 increases as a function of time.
Consequently, if the pilot actuates the control member at a low speed Vr, the time T required to reach the speed Vlof is large so that, the attitude angle xcex8 increasing as a function of time, it is possible to reach a value of attitude angle xcex8 which is very high at the time tr+T, and which may be such that the rear part of the fuselage touches the ground.
Moreover, to reduce the takeoff distance, the pilot may be obliged to take off with minimum speeds Vr and Vlof, this requiring the nose of the aircraft to be raised further thereby increasing the attitude angle. Of course this heightens the risk of touching the ground with the rear part of the fuselage.
This risk is all the greater the longer the aircraft and hence the bigger the distance L between its main gear and said rear part of the fuselage. Specifically, for a given height of gear, the critical angle xcex8q for which the rear part of the aircraft touches the ground decreases as the distance L increases. For example, on an aircraft of the xe2x80x9cAirbus A340xe2x80x9d type, the critical angle xcex8q is of the order of from 13.5xc2x0 to 14xc2x0, the value of xcex8 during takeoff with a minimum speed Vr being of the order of from 12xc2x0 to 12.5xc2x0 depending on the configuration of the aircraft. As a result of this, the margin of the attitude angle xcex8 at takeoff is of the order of only 1.5xc2x0. The risk of touching the ground with the rear part of the fuselage and hence of damaging the aircraft must consequently be taken into consideration.
Patent FR-2 711 257 discloses a control system for elevators of an aircraft comprising means making it possible to attenuate the control signal of said elevators, as a function of the attitude angle xcex8 of the aircraft. The attenuation increases, up to a specified threshold, as the angle xcex8 increases. By attenuating the increase in xcex8 as a function of time, such a system reduces the risk of touching the ground with the rear part of the fuselage.
However, this risk is not completely eradicated. Specifically, despite this attenuation, for a very large time T, the attitude angle xcex8 may reach the aforesaid critical value xcex8q. Moreover, this known system has the drawback of attenuating the rate of rotation (or variation of attitude as a function of time) in all cases, even when the speed Vr is high enough for there to be no risk of touching the ground with the rear part of the fuselage. In this case, this known system needlessly lengthens the time required to reach the value of the attitude angle xcex8 allowing takeoff to be achieved, this obviously having the effect of increasing the distance required for takeoff.
The present invention relates to a process for controlling at least one aerodynamic elevator surface of an aircraft, during a takeoff phase, which makes it possible to remedy the aforesaid drawbacks.
To this end, said process is noteworthy, according to the invention, in that, during the acceleration of the aircraft running along the ground with a view to takeoff, as soon as a pilot actuates a control member for controlling said aerodynamic elevator surface in such a way as to increase the pitch attitude angle of the aircraft with respect to the ground, at a ground running speed Vr:
a) a speed Vlof min is determined at which the last wheels of the aircraft leave the ground in the case of a takeoff for which the pitch attitude angle is controlled according to a nominal control law and exhibits at this speed Vlof min a nominal value xcex8nom which is below, by a predefined margin, a critical value xcex8q of pitch attitude angle, for which the aircraft touches the ground at the rear with a part of its fuselage;
b) the duration T required for the aircraft to go from the speed Vr to this speed Vlof min is determined;
c) this duration T is compared with a nominal duration Tnom which corresponds to the duration required for the aircraft to go, according to a nominal control law, from a pitch attitude of running xcex8r (at the speed Vr) to said pitch attitude of nominal value xcex8nom; and
d) from this comparison is deduced a control law for the pitch attitude angle, which is applied to the aircraft and which corresponds:
if the duration T is less than or equal to the nominal duration Tnom, to said nominal control law; and
if the duration T is greater than the nominal duration Tnom, to a modified control law which is such that at a duration T after the actuation of the control member by the pilot, the pitch attitude angle of the aircraft is substantially equal to said nominal value.
Thus, by virtue of the invention, when there is a risk of the fuselage of the aircraft touching the ground when the aerodynamic elevator surface is controlled by a nominal (theoretical) control law, a modified law which prevents such contact is applied to said aerodynamic surface.
Consequently, there is no longer any risk of contact, this of course being a considerable advantage in respect of safety.
Moreover, unlike the solution proposed by the aforesaid patent FR-2 711 257, the nominal control law is modified only when actually necessary for safety. In the converse case, the aircraft is still controlled according to the nominal control law which has the advantage (as will be seen hereinbelow) of representing the actuation of the control member by the pilot more faithfully.
Preferably, said modified control law substantially follows the nominal control law for a duration of 3T/4 starting from the actuation of the control member by the pilot, then deviates from this nominal control law so that at the duration T the pitch attitude angle of the aircraft is substantially equal to said nominal value.
Thus, for a major part of the time lying between tr and tr+T, the modified control law remains close to the nominal (theoretical) control law. The advantage of this for the pilot is that he feels, for as long as possible, the effect of his action on the control member.
Additionally, advantageously, the state of at least one parameter of the aircraft which is capable of taking one of at least two states is determined and, for each of the states of said parameter, a different nominal control law and a different modified control law are used. In this case, when the aircraft is fitted with at least two engines, said parameter relates, preferably, to the operation of said engines, one of the states corresponding to normal operation of all the engines and the other state corresponding to a failure of one of said engines.
According to the invention, in the aforesaid step a), the duration T is determined as a function of parameters of the aircraft, of the acceleration of the aircraft at the moment of the actuation of the control member by the pilot, and of the state of the engines of the aircraft. This can be achieved:
by solving the following equation:
Vlof minxe2x88x92Vr=∫trTAcc.dt
xe2x80x83in which:
tr is the moment at which the pilot actuates the control member; and
Acc represents the acceleration of the aircraft; or
from the following simplified expression:
T=(Vlof minxe2x88x92Vr)/Amoy,
Amoy being the average acceleration of the aircraft between the speeds Vr and Vlof min. This average acceleration Amoy is preferably determined by multiplying the acceleration which is measured at the moment at which the pilot actuates the control member, by a predefined coefficient k which is less than 1.
Additionally, advantageously, the speed Vlof min which is used to determine the duration T is determined as a function of the mass of the aircraft, of its configuration, of the state of its engines and of its environment.
The present invention also relates to a device for controlling at least one aerodynamic elevator surface of an aircraft during a take off phase.
According to the invention, said device is of the type comprising:
a control member, for example a ministick, which is capable of being actuated by a pilot and which is associated with a means (position sensor for example) delivering a signal which is representative of such an actuation;
calculation means for determining, at least from said signal, a control low; and
motor means, for example an electric motor or a ram for controlling said aerodynamic elevator surface as a function of a control law received from said calculation means, is noteworthy in that said motor means control said aerodynamic elevator surface at least between the moment of a first actuation of the control member by the pilot so as to control said aerodynamic elevator surface in such a way as to increase the pitch attitude angle of the aircraft with respect to the ground at a ground running speed Vr and the moment at which the last wheels of the aircraft leave the ground at a running speed Vlof, and in that said calculation means comprise:
means for determining the duration T required for the aircraft to go from the speed Vr to the speed Vlof min;
means for comparing this duration T with a nominal duration Tnom which corresponds to the duration required for the aircraft to go from a running pitch attitude xcex8r (at the speed Vr) to an attitude of nominal value xcex8nom during a nominal takeoff, for which the pitch attitude angle is controlled according to a nominal theoretical control law, this nominal value xcex8nom being below, by a predefined margin, a critical value xcex8q of pitch attitude angle, for which the aircraft touches the ground at the rear with a part of its fuselage; and
means for deducing from this comparison the control law for the pitch attitude angle, which is transmitted to the motor means, this control law corresponding:
if the duration T is less than or equal to the nominal duration Tnom, to said nominal control law; and
if the duration T is greater than the nominal duration Tnom, to a modified control law which is such that at a duration T after the actuation of the control member by the pilot, the pitch attitude angle of the aircraft is substantially equal to said nominal value.